This is a competing renewal application for continuation of studies to further enhance our understanding of electrolyte transport across the gastrointestinal tract during the infancy period. Specifically the studies are designed to investigate the molecular mechanism underlying the ontogenic changes in two apical sodium-hydrogen exchangers (NHE2 and NHE-3), which are highly expressed in the gastrointestinal tract. The physiological importance of NHE-2 and NHE-3 has been documented by the phenotypic changes seen in mice lacking those isoforms. Our studies during the last funding period have defined the ontogenic changes in NHE-2 and NHE-3 at the molecular level and suggested that their regulation occurs at the transcriptional level. These studies form the foundation for the proposed studies. Preliminary studies in our laboratory suggested an important role for epidermal growth factor (EGF) and short chain fatty acids (SCFA's) in the regulation of NHE-2 and NHE-3 respectively. To further expand these studies, we plan to test the hypothesis that transcriptional regulation is critical for the ontogenic changes seen in NHE-2 and NHE-3. To test this hypothesis we plan to pursue three specific aims: Specific aim 1 is designed to identify and characterize ciselement and transacting factors involved in NHE-2 and NHE-3 mammalian intestine. Specific aim 2 is designed to decipher the molecular mechanisms of transcriptional regulation of NHE-2 by EGF and NHE-3 by SCFA's. Specific aim 3 is designed to identify and characterize cis-elements and transacting factors involved in the ontogenic change seen in NHE-2 and NHE-3 gene expression. State of the art molecular tools will be utilized to address the three specific aims. The proposed studies will significantly enhance our understanding of the transcriptional regulation of these two important NHE's during early life. The findings will have significant relevance to further enhance our knowledge in understanding the pathophysiology of diarrheal disorders in the infancy period, which causes significant morbidity and mortality worldwide.